Stabilization process within a sewage sludge landfill determined through both particle size distribution and content of humic substances as well as by FT-IR analysis

Landfill is largely considered as a reliable option for sewage sludge disposal in most metropolitan areas worldwide due to the huge quantities of this waste to be disposed of and the relatively low costs of such a kind of sludge management. It has been found that the sludge in the landfill degrades rapidly and becomes stabilized within a few years. In the present study, the sludge from different landfill stages was characterized by particle size distribution, humic substances contents and elemental composition, and Fourier transform infrared spectroscopy (FT-IR), as the landfill time increased. In general, the mean particle size of the sludge increased from 37 μm at day 0 to 143 μm at 300 days and the corresponding median particle size increased from 13 to 70 μm. The stability of particle size distribution can be assessed by the mean or median particle size. The humic acid (HA) and fulvic acid (FA) contents extracted from dry sludge after different landfill periods increased from 4.2 and 2.7% of day 0 to 5.6 and 3.1%, respectively, at 400 days, thereby indicating that the stabilization process of sludge in a landfill is also a humification process. The HA samples contained more carbon and nitrogen, and less hydrogen and oxygen than the FA samples, indicating a high degree of maturity and humification of these HA samples. The FT-IR spectra indicated that easily degradable organic matter components, such as aliphatic chains and protein, were distinctly decomposed during landfill. Based on the changes in the band relative intensity, it was concluded that after 300 days in a landfill the sludge is still in the process of degradation and maturity.